Study suggests Southwest fires driven by year-to-year weather cycles rather than long-term climate variations

A low-intensity ground fire burns through ponderosa pine during a prescribed fire in Grand Canyon National Park. PHOTO COURTESY NATIONAL PARK SERVICE.

New study offers more evidence that historical fire suppression is the main contributing factor in today’s big southwestern fires

By Summit Voice

SUMMIT COUNTY — A detailed analysis of historic fire-scar records and tree-ring data going back centuries offers more evidence that recent mega-fires in the Southwest are unprecedented and likely a result of widespread fire suppression.

While many of today’s southwestern forests haven’t seen a fire in more than 100 years, historical records show that, in the pre-suppression era, they rarely went as long as 40 or 50 years without a low-intensity ground fire.

To reach their conclusions, researchers at Southern Methodist University used a statistical model that encompassed 1,500 years of climate and fire patterns. They wanted to test if today’s dry, hot climate alone is causing the megafires that routinely destroy millions of acres of forest.

The results were striking. Year-to-year weather patterns that drive fire activity seemed to stay the same against the backdrop of larger climate cycles, according to SMU fire anthropologist Christopher I. Roos.

“The U.S. would not be experiencing massive large-canopy-killing crown fires today if human activities had not begun to suppress the low-severity surface fires that were so common more than a century ago,” said Roos, an assistant professor in the SMU Department of Anthropology.

The study isn’t the first to suggest that livestock grazing, firefighting and other activities resulted in dense forests with accumulated logs and other fuels that now make them more vulnerable than ever to extreme drought. One answer to today’s megafires might be changes in fire management.

“If anything, what climate change reminds us is that it’s pretty urgent that we deal with the structural problems in the forests. The forests may be equipped to handle the climate change, but not in the condition that they’re currently in. They haven’t been in that condition before,” Roos said.

Roos worked with the University of Arizona’s Thomas W. Swetnam, an eminent dendrochronologist, to publish the findings in the scientific journal The Holocene. The study focused on ponderosa pine forests of the Southwest.

Other studies examined forest fire records from the Little Ice Age, about 200 to 400 years ago, to try and understand current fire behavior, finding that there were frequent low intensity fires burning on the surface, but not in the tree canopy.

That research was challenged by other scientists who said that the comparison was not useful because the Little Ice Age climate was cooler and wetter than the current period. A more valid comparison might be the Medieval Warm Period (800 to 1300) when the climate was more like today’s.

So Roos and Swetnam decided to put those comparisons to the test with a model that combined 200 years of Little Ice Age fire-scar data and nearly 1,500 years of climate data derived from existing tree rings — and finding that the Medieval Warm Period was no different from the Little Ice Age. During both periods, the main driver of fire behavior is year-to-year moisture patterns.

“It’s true that global warming is increasing the magnitude of the droughts we’re facing, but droughts were even more severe during the Medieval Warm Period,” Roos said. “It turns out that what’s driving the frequency of surface fires is having a couple wet years that allow grasses to grow continuously across the forest floor and then a dry year in which they can burn. We found a really strong statistical relationship between two or more wet years followed by a dry year, which produced lots of fires.”

Scientists who favor that comparison hypothesize that forest fires during the Medieval Warm Period probably were similar to today’s megafires and probably more destructive than during the Little Ice Age.

Scientists rely on tree rings not only to calculate a tree’s age, but also to determine wet and dry weather patterns of moisture and drought. Similarly, scientists’ best evidence for fire activity is the scarring on tree rings that dates the occurrence of fires. While tree-ring data for climate are available for long time periods, annual forest fire records don’t yet exist for the Medieval Warm Period.

“The best way to look at how fires may have varied — if climate were the only driver — is to do this type of modeling,” Roos said. “Our study is the first in the world to go this far back using this methodology. But this method can be used anyplace for which there is a fire-scar record.”

The study’s tree-ring-derived climate data are from the southern Colorado Plateau, a region that includes the world’s largest continuous stand of Ponderosa Pine stretching from Flagstaff, Ariz., into New Mexico. Large Ponderosa Pine forests have existed in the area for more than 10,000 years.

Fire-scar data for the region go back as far as the 1500s, but are most prevalent during the Little Ice Age period. Fire scientists have analyzed fire-scars from hundreds of trees from more than 100 locations across the Southwest. All fire-scar data are publically available through the International Multiproxy Paleofire Database, maintained by the federal National Oceanic and Atmospheric Administration’s paleoclimatology program.

The fire records show that frequent fires burned on the forest floor both during warmer and cooler periods, driven by much shorter cycles of precipitation.

“The fires cleaned up the understory, kept it very open, and made it resilient to climate changes because even if there was a really severe drought, there weren’t the big explosive fires that burn through the canopy because there were no fuels to take it up there,” Roos said. “The trees had adapted to frequent surface fires, and adult trees didn’t die from massive fire events because the fires burned on the surface and not in the canopy.”

The pattern started changing in the late 1800s, during the settlement era, when ranchers started altering the composition of the forest landscape. Grazing animals ate grasses that once fueled small fires, but left saplings and brush, which started to grow into dense, mature bushes and trees.

Government-imposed restrictions on traditional land uses by the region’s Native American communities may have been another factor, eliminating another source of surface fires. When the Forest Service started focusing on firefighting in the early 20th century, the cycle of suppression was complete. Without continuous fuel, fires on the forest floor ceased.

“Many of our modern forests in central Arizona and New Mexico haven’t had a fire of any kind on them in 130 or 140 years,” Roos said. “That’s very different from the records of the ancient forests. The longest they would have gone without fires was 40 or 50 years, and even that length of time would have been exceptional.”

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